Abrasive grains, in particular ceramic abrasive grains, with a defined form and size have been known for some time.
U.S. Pat. No. 5,201,916 discloses inter alia flat abrasive grains with for example a trianglular, rectangular or circular form. These abrasive grains are produced from a dispersion which contains particles that can be converted into α-alumina and a liquid with a volatile component. The dispersion is poured into a casting mold, which has a planar base area and depressions, the forms of which are complementary to the desired forms of the abrasive grains. Subsequently, part of the volatile component is removed, and so a precursor with the desired form is produced. The precursor is then removed from the mold, calcined and finally sintered so as to produce the finished abrasive grain.
The abrasive grains produced by this method have two opposite main faces, which have substantially the same geometrical form. The abrasive grains are given a longer lifetime, since small pieces continually break off from the abrasive grains during grinding, and so new cutting surfaces are obtained. The abrasive grains thereby sharpen themselves. It is presumed that, in the case of the abrasive grains with a base area in the form of a triangle, in particular an equilateral triangle, with electrostatic scattering approximately one to two thirds become oriented in such a way that one vertex faces away from the backings, while other abrasive grains orient themselves in such a way that the vertex faces the backing.
In an alternative method, described in EP 615 816, elongate, rod-shaped precursors are first produced by means of extrusion, and then divided into individual abrasive grains. The rod-shaped abrasive grains may consequently have for example the form of a cylinder or prism.
In WO 2009/085841 there is a description of a further method of production in which the precursor is dried in the casting mold under conditions that lead to fracturing of the precursor. The shards at least partially have surfaces and edges that are complementary to the corresponding surfaces and edges of the casting mold, and therefore have the angle defined by the casting mold. These surfaces and edges provide an enhanced cutting capability. On the other hand, the further surfaces and edges produced by the fracturing are irregular.
WO 2010/077495 discloses abrasive grains which contain openings passing right through or not passing right through or have a dish-like form. Methods of production for such abrasive grains are also described there. Further abrasive grains with undefined openings are disclosed in WO 2010/077518. WO 2010/077491 is likewise concerned with abrasive grains with a dish-like form.
WO 2010/077519 discloses abrasive grains that have two opposite main faces and side faces that extend between them and are inclined in relation to the main faces. The various side faces of an abrasive grain may be inclined at different angles in relation to the main faces.
The document WO 2011/068724 likewise shows abrasive grains that have a base side and a vertex and also inclined side faces extending in between. Similar forms of abrasive grain are also described in WO 2011/109188.
The document WO 2010/077509 is concerned with abrasive grains that have a surface with a multiplicity of grooves. These grooves are produced with the aid of complementary ridges on the underside of the casting mold.
WO 2011/068714 shows pyramidal abrasive grains with a parallelogram-shaped, in particular rhomboidal, base area, a kite-shaped base area and a superelliptical base area.
WO 2011/139562 discloses abrasive grains in the form of tetrahedrons and modifications thereof. For example, the side faces may be concavely or convexly formed, the corners of the tetrahedron may be truncated, or the edges may be curve-shaped.
The abrasive grains described in WO 2012/018903 include two or more plate-shaped portions, which are arranged at an angle in relation to one another.
In the case of the method described in WO 2012/061016, firstly an abrasive formation is produced, containing abrasive grain precursors that are connected to one another by way of frangible webs. After sintering, the abrasive grains are separated from one another by the webs being severed.
Alternatively, abrasive grains of a defined form may also be produced by a screen printing process. This is described for example by WO 96/12776. In this case, a dimensionally stable dispersion is passed through openings with a defined form onto a transporting belt and subsequently cured. The openings may for example be contained in a movable endless belt.
A further development of the screen printing process is disclosed in WO 2011/087649. In the case of this process, the dispersion is forced through the openings in the endless belt by means of a differential pressure. Given a suitable choice of the viscosity of the dispersion, with this process it is possible to produce abrasive grains of a cross section that tapers from a first main side to a second, opposite main side.
In WO 2012/061033 there is a description of methods for producing abrasive grains of a defined form with the aid of laser radiation. Moreover, further special forms of abrasive grains are disclosed. For example, the abrasive grains may include a main element and at least three rod-shaped elements extending from it. In particular, the abrasive grain may have the form of a cross, an uppercase letter “T”, a star or a lowercase Greek letter “λ”.
It is generally assumed that abrasive grains with a defined form have improved properties from several aspects: if the abrasive grains have a defined form and size right from the beginning of their production, there is no need for a subsequent sorting step, by which the abrasive grains would otherwise have to be divided into different size fractions. Moreover, the forms and sizes also remain virtually unchanged between different production batches, which makes the abrasive properties reproducible very well. Furthermore, the abrasive grains may for example provide an increased overall removal, have a longer lifetime, produce an enhanced surface quality of the surface worked or provide a better-reproducible grinding result.
Nevertheless, the abrasive grains known from the prior art have a series of disadvantages. For example, many of the known abrasive grains cannot be arranged on the abrasive material backing in a very space-saving way. Moreover, many of the known abrasive grains cannot be anchored sufficiently in a binder applied to an abrasive material backing.